When producing elastomeric compositions for use in rubber articles, such as tires, power belts, or vibration isolation components, it is desirable that these elastomeric compositions are processable during compounding. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, modulus, viscoelastic properties such as hysteresis loss (tan δ), and wear resistance.
Mixing silica into rubber stocks, however, presents challenges because silica particles containing polar silanol groups on the surface tend to self-associate and reagglomerate extensively after compounding, leading to poor silica dispersion and a high compound viscosity. The strong and highly developed silica filler network results in a rigid uncured compound that is difficult to process in downstream processes, such as extrusion and curing operations.
Silica shielding and coupling agents have been developed to alleviate this problem; however, a feature of conventional silica dispersing agents is the presence of one or more alkoxysilane groups that react with the silanol groups on the silica surface (the silanization reaction) during mixing of the rubber compound. This reaction causes the evolution and release of alcohol into the environment. In particular, when the mixing is conducted at high processing temperatures, alcohol is released and contributes to the volatile organic compounds (VOCs) generated during processing of the rubber compounds. At lower processing temperatures, the compounded product can retain a considerable amount of unreacted alkoxysilyl groups that are available to further react with the silica and moisture during storage, extrusion, tire build, and/or curing, resulting in an undesirable increase in the compound viscosity, a shorter shelf life, and the associated porosity issues. This continuing reaction in the compounded product evolves additional alcohol that may impair further processing of the compound and final product quality. As a result, a decreased extrusion rate of the resulting compounds may be utilized to ensure the drawn composition conforms to specifications. In addition, special attention during curing is given to prevent the porosity formed in the final product. These methods of dealing with the VOC generation result in decreased production and concomitant increase in cost.
Several approaches to reduce VOC evolution have been tried, but the shielding level produced was not as good as that which could be obtained with organosiloxanes. The organosiloxanes can cause an evolution of up to 25% of their initially added weight as ethanol when mixed with rubber in an initial mixing step, followed by an additional 15% during the curing stage, followed by an additional 10% slowly being evolved during use.